1. Field of Invention
This invention relates to powered parachutes, powered gliders, and ultra-light aircraft having Vertical Take-off and Landing (VToL) capability.
2. Background of the Invention
Prior art flying platforms and small helicopters have sought to provide a VToL Personal Flying Vehicle as a practical recreational vehicle, but have not sufficiently satisfied issues of safety—especially regarding engine failure and un-powered descents, and over-complexity of the control system, which render the prior art air vehicles unsuitable for recreational use.
The ‘Backyard Flier’ Concept:
The phrase “backyard flyer” is used frequently to describe the ideal vehicle, reflecting the desire for one that does not need a runway or special facilities, but can be operated from home. This infers a vehicle of diminutive size—something that might be parked in the driveway, alongside the boat, ATV, and other recreational vehicles. Vertical take-offs are a prerequisite for such a vehicle—but it is the ability to descend vertically, in a highly controlled manner, that gives the personal flyer its magic appeal, for anyone who has spent any appreciable time traversing the earth's vast regions of remote backcountry will intuitively recognize the benefit that such freedom to land almost anywhere confers.
A particular concept of ‘personal flight’ has evolved over time to include: “the ability of a person to fly about freely, in three dimensions, to take-off and land vertically, with a minimum of artificial contrivance.” That such a vehicle must, especially where human operators are concerned, be made as safe as possible, is a given. The predominant answer to the problem of safety in the art has been one of over-engineering of the primary propulsion system, and the provision of various “back-up” systems. Some designs provide for secondary emergency-descent system, such as a ballistic parachute. These measures have been deemed insufficient by the public because to date, no vertically-flying personal aircraft have gained wide acceptance in the recreational marketplace. However, much was learned in the “motorized hang-glider” era, and the present-day phenomenon of “motorized parachutes” makes a strong case that simple, ‘minimalist’ aircraft will be accepted as a popular, recreational craft if the vehicle is perceived as having a high safety-quotient. The ‘low and slow’ Flying Parachutes, “Trikes”, etc., have proven to be practical, reasonably safe, (continued . . . ) and of high utility in a number of applications besides sport aviation. However, the ultimate ‘backyard flier’ is, by definition, a Vertical Take off and Landing (VToL) one, and even the best powered parachutes require a take-off run and clearance over obstacles. The question raised is how to incorporate the ‘intrinsic safety’ of the powered parachute into a VToL craft having reasonably small dimensions. In this air-vehicle invention, resolution of the problem of a small personal VToL flyer is made by applying several concepts borrowed from the early ‘hang-glider” technology, especially in regards to weight-shift control, light wire-braced framing, and use of composites. It benefits from the improved thrust/hp ratio of modem power plants, and of the new “dual-vortex’ propulsion system, described in a separate Utility patent application, currently being prepared. This air-vehicle invention also benefits from a novel application of a multiplicity of rotatable louver-vanes, placed radially above and below a disk of counter-rotating blades, acting as a descent-retarding mechanism, as described herein.
Flying Platforms:
(Non Patent Literature: (2) Hiller Aviation Museum—Zimmerman excerpt, 2 pages (3) R. Paul Hill biography, wikepedia, 4 pages (4) Synopsis of “Unconventional Flying Objects” by Paul Hill, 4 pages)
Another feature in the prior art of value in the search for a safe personal air vehicle is the weight-shift control concept, as applied to the flying-platform type of vehicle. Research started in 1951 by Dr. C. Zimmerman, and furthered by NASA engineer Paul Hill, proved that for small, hovering vehicles, fine vernier directional control of the craft is most simply accomplished by the pilot shifting his or her weight in relation to the crafts' normal Centre-of-Gravity (C of G). The “craft” was a simple platform to stand on, to which a thrusting device is attached, in a fixed position, with the thrust directed downward. This research led to flying platforms for the military, and experimental vehicles for the Lunar Rover program. The standing position was discovered to be the best, and it was postulated that man's long evolution of bi-pedalism resulted in a matchless capacity of humans for upright balance. This skill is transferable to operations in three dimensions as well as the usual two, a fact possibly accounted for by man's 3-d, arboreal locomotion in our distant past. But flying platforms have a major stability problem; being top-heavy, they are ‘tippy’ and can over-rotate to an inverted position when tilted too far. U.S. Pat. No. 2,953,321, the Hiller Flying Platform, is typical of this category in its use of a deployable ballistic parachute as a means of secondary, emergency descent. However, problems include the possibility of the vehicle being at too low an altitude for adequate time for the parachute canopy to inflate. It also depends on the pilot being capable of initiating the deployment—which may not be the case. The pilot may be incapacitated or momentarily frozen by the stress of the moment. These parachutes provide no control over the descent, which may land the vehicle dangerously, in a body of water, busy highway, etc. Since rotors aren't efficient at developing lift than are fixed wing aircraft, flying platforms are less fuel-efficient, with less range.
Safe Aerodynamic Descent:
An aircraft designed for safe hovering and low-and-slow flight requires a sturdy pilot enclosure and a reliable secondary means of a slow, controllable, power-off descent. In other words, if an exclusive reliance on auto-rotation is replaced by reliance upon an intrinsic, aerodynamic capacity of the vehicle-as-a-whole, the vertically-flying vehicle can be made safe. To date, no ultra light VToL air vehicle designs have attempted to modify the aero-form itself, to serve as a secondary means of descent in the event of propulsion-system failure.
Additional Requirements of a Personal Air-Vehicle:
For the prospective casual pilot—one who intends only recreational, weekend use, there are other concerns that also involve safety. Ideally, The field has benefited from new, high power-to-weight motors and modern ultra-light aircraft construction Methods and Materials. This affords the possibility of new types of VToL one and two-place ultralights that are relatively inexpensive. These three objects—easy-to-learn, affordablility, and with a built-in, intrinsically safe descent, would put such a VToL Flyer in the recreational category, with significant market potential.
Personal Helicopters:
Personal helicopters are a growing market, but require a substantial commitment in cost and training time required, and will never, for these reasons, become the ‘popular backyard flyer’. A wide-diameter open rotor, mechanical and operational over-complexity, and no substantial means of secondary descent in the event of failure of the primary rotor to auto-rotate, all conspire to make the conventional helicopter a poor candidate as a mass-market VToL flier. Casual users are unwilling to commit the time necessary for initial and ongoing refresher training. With a rotor diameter of 9 meters or more, even the smallest helicopters require a substantial space for landing and taking-off. Rotor tip-strikes against obstacles near the ground is the most common causes of helicopter accidents. They have limited lift, and are too expensive for the average person. Since rotors aren't as efficient at producing lift than a fixed-wing, helicopters are less fuel efficient, with less utility and cross-country capability.
Parachute-Wings:
To date, the motorized parachute-wing, or para-glider, and the motorized Rogallo-wing, also known as a ‘trike’ are the only two aircraft in the recreational category that have an intrinsically safe descent capability. The Rogallo-wing is not a true wing and does not create lift as does a true, airfoil-section wing. Instead, it relies on the “kite effect” to arrest descent, which involves presenting a flat surface of sufficient area to the oncoming airflow. Thus, while it is not capable of a vertical take-off, Rogallos can be made to descend vertically, or nearly vertically, with very little forward motion. Para-gliders, or para-sails or parachute-wings, do employ an airfoil shape to create lift, but this only occurs when the wing has a forward motion, relative to the air, which is higher than the wings' stall speed, which is usually quite slow, around 5-10 kph. Below its stall speed the para-glider does not use lift to arrest its descent, but employs the kite-effect, like the Rogallo, and also the “parachute effect”. The parachute effect occurs when, during its vertical descent, the upwardly-convex undersurface of the parasail captures a bubble of air, in the same manner as a conventional, hemi spherically-shaped parachute. Thus, unlike a conventional airfoil-section wing with a flat bottom surface, the highly upwardly-convex Rogallo-wing and the parachute-wing both provide an acceptably slow vertical descent with little or no forward motion. Para-sails or para-glider types are customarily non-rigid, constructed from fabric and using ambient airflow to inflate pockets, or cells, to attain its wing-like shape. Rogallos are semi-ridgid, with inflated fabric pockets on a triangular plan-form frame. Semi-rigid and rigid construction modes are possible with this air-vehicle invention, using composites and other lightweight materials. Some rigid, fixed-wing ultra light aircraft have also incorporated wings with such a highly (continue . . . ) cambered airfoil section that, upon descent, they exhibit the parachute effect. Flying-wing, ultra-light motor-gliders, including the Mitchell Wing A-10, are able, when speed is reduced below the stall, to transition to a parachute-effect vertical descent that remains controllable. It stands repeating however, that while both the Rogallo and parasail-types are capable of near-vertical descents, the motorized versions require a large enough take-off area to facilitate a take-off run of some appreciable distance. Thus, para-glider type aeroforms are capable of slow vertical descents, by virtue of their intrinsic, aerodynamic shape, using the kite-effect and the parachute effect. A slow landing is a built-in feature of this aero form; it is part of the airframe and is available full-time, regardless of the status of the main propulsion system. In summary, the para-glider is not capable of vertical take-offs, like a helicopter. In normal forward flight the canopy or rigid parachute-wing does act as a true wing, which renders the aircraft more efficient in lift and therefore more fuel efficient than a helicopter. In the event of a motor failure, the parachute wing offers a respectable glide ratio up to approximately 3 or 4 to one, is controllable, and has acceptably low landing speeds.
A noted feature of the motorized paraglider is its capability of a slow vertical descent, with or without a functioning propulsion unit. The descent system is deployed full time and requires no action on the part of the pilot during an emergency descent. Safety is what has made motorized paragliding the popular sport it has become. As noted, although the nominal wingspan of this type, at around 9 meters, is suitable for vertically taking-off from a confined space if it were so able, its inability to lift vertically and the need for a large area for take-offs is a big drawback. The addition of VToL capability would make this already popular form of recreational flying available to more people. A paraglider-type air-vehicle with VToL capability offers enhanced flying experience with the added dimension of hovering and precise directional control.
One solution to giving a para-glider aero-form VToL capability was the ArcWing, U.S. Pat. No. 415,131. It is a deflected-thrust type of lifter, but instead of using independent deflector flaps, as previous types had done, the ArcWing uses the main parasail itself as the deflector. Transition from vertical to forward normal flight is by mechanically altering the position of the wing, simultaneously changing its angle-of-incidence relative to the horizontal thrust produced by the vertically-mounted propeller. Like other deflected-thrust types, the ArcWing suffers the major disadvantage of loss of thrust during the re-directing process, requiring more powerful, heavier motors for vertical take-offs.
Aircraft Using Parachute-Wing/Descent-Vanes with Horizontally-Mounted Propeller, No Existing Classification Found:
Other examples showing personal aircraft using a down-thrust, or horizontally-mounted rotor to propel a weight-shift controlled paraglider are not known. If deemed a new type, criteria may include: 1) Full-time, parachute-like aerodynamic descent, using descent appendages, vanes, panels, wings, etc., and capable of a stable, controllable vertical descent; 2) a powered rotor for vertical take-off and landing, & hover; 3) In forward flight, the attached descent elements—descent sails, descent vanes, descent wings, etc. also produce lift, for better efficiency and higher top speeds.
Ducted-Fan Aircraft, Tiltrotors, Winged Rotorcraft:
Many rotor and ducted-fan arrangements are known, and include: tilt-rotors, tilt-wings, circular disk-wings, and variants with wings in the monoplane, biplane, and multi-plane configurations. The art contains winged lifting-disks, or winged ducted propellers, of which U.S. Pat. No. 7,281,680, by inventor Melkuti, is a recent example. The wings are small relative to the crafts' overall plan-form, and have an airfoil section selected for efficiency in the mid and upper speed range of the vehicle. Therefore the glide characteristics are poor, with a relatively high stall-speed, low glide ratio, and fast rates of descent. Invariably, the wings mounted on ducted rotor vehicles have been true wings, which have the primary purpose of providing lift in normal forward flight. None are configured to affect low-speed descents and, as we have seen, while certain types of true wings can be used as effective descent elements, so can a flat panel, a cambered surface, or some other form of appendage. In comparison, the parachute-glider type of aero form, that is especially designed for both gliding and vertical (parachute) descents, have sufficient drag-producing area to arrest an emergency descent—besides providing efficient lift during normal forward flight as well. For the ‘true-wing’ configurations, employed on the ducted-fan, tilt-rotor types, etc., the object of a slow, vertical aerodynamic descent has not been claimed.
Flying platforms typically have a shroud or duct surrounding the rotor for safety and to improve efficiency. The top inlet of this duct is flaired outward to create lift and to improve the smooth inflow of air. Robertson (U.S. Pat. No. 2,953,321) is a good example. It has been periodically suggested that by expanding the curved top edge into an annular airfoil, it would act, during an unpowered descent, like a parachute. In fact this idea was fully developed 85 years ago by Polish Air Force Captain, A. Sippowicz (Non-Patent Literature (1) pg. 32 and 33, excerpts from “UFO's and Antigravity’ By Leonard Cramp, 6 pages)—but it has drawbacks, especially if applied to a platform-type vehicle where the pilot is standing. While Sippowicz's 1928 Helipan can produce a slow unpowered descent, it also renders the vehicle incapable of horizontal flight at speed, or operation in anything more than a light breeze. Unlike a helicopter rotor-disk, which operates within a column of air of its own creation, and can develop lift and thrust whichever direction it is oriented in, the same is not true of a flat or curved surface, such as a Helipan-type annular ring-airfoil, because it cannot create lift at a negative pitch of more than 2 or 3 degrees. Tilted further, the down force on the top surface overcomes lift, requiring more and more power to maintain altitude, because flying platforms can only increase horizontal speed by increasing pitch, which produces more drag, and so on, and so is self-defeating. The only way to overcome this defect would be to not pitch the flying platform to initiate horizontal flight, but instead rely on some form of auxiliary power to propel it horizontally. The platform could not then be weight-shift controlled however. Perhaps the enlarged inlet-ring could be set at a predetermined angle, or made rotatable in relation to the plane of the rotor, but this would then interfere with its primary function —the efficient (and symmetrical) inflow of air. An annular ring of a diameter sufficient to arrest a descent would also occlude the downward view of the pilot. In any case no-one has claimed or attempted such a “descent-ring” on a platform. Insofar as flying platforms go, the main disqualifying flaw in the descent-ring solution is that, with the pilot standing, the duct inlet will always be located well below the vehicles' centre-of-gravity.
This is of no consequence when under power, when the inlet is functioning as an inlet and the craft is supported on a column of air. But, for an unpowered descent, this location for a descent-arresting airfoil is catastrophic. Being top-heavy, tip-overs are the only possible outcome. This is especially true the bigger or more curved the ring is, since a curved surface always inclines towards the direction of least resistance—towards the direction it is curved in, thus forcing a rapid turn-over of the vehicle every time (and which I have personally empirically tested using models). My descent-vane system, in contrast, has airfoils that are engaged with the airflow at the correct angle for sustained horizontal flight at speed and, being placed high above the C of G, or at the very top of the aircraft, no centre of gravity or turn-over problems are encountered. In fact, the top position is the ultimate where an unpowered descent is concerned, because the “pendulum effect” gives the platform the maximum possible stability during an unpowered descent.
Winged Helicopters:
Improvements in the power/weight ratios of small gas engines, and the increasing affordability of powerful, small jet turbines has given new impetus to the VToL field. As a class, small one and two-man helicopters have had the most success. While no winged ultra light helicopters are known, some experimental and military helicopters incorporate lifting wings in configuration with one or more lifting rotors, in so-called “hybrid” configurations. Where employed, examples of this design strive to gain efficiency at the higher speeds, and produce faster top speeds, and the wings have little or no effect in an un-powered descent mode.
In the event of an engine failure of a conventional helicopter type, a gliding-type landing is attempted by the pilot by a process of autorotation, but this requires much training. Damage to a rotor is a more serious problem, because there is no opportunity for any secondary descent capability, such as a ballistic parachute. Because of these limitations helicopters are best left to the professional/commercial field. Because of the complexity of their control system and the high training requirement, helicopters of a conventional configuration are unlikely to become widely popular as a recreational flyer.
Emergency Descent of Flying Platforms:
Flying Platforms are another subclass of the VToL field, which has benefited from improvements in small, lightweight motors. Here, the payload or occupant is located on a small platform centered above the lifting rotor. Contra-rotating rotors have frequently been used, in order to defeat the problem of torque and the need of a tail rotor. Unfortunately, it was difficult to engineer a co-axial, counter-rotating rotor system that is also capable of auto-rotating and, being generally of the ducted variety, these rotors are of too small a diameter to effectively auto-rotate in any case. Unlike a helicopter however, the platform can be equipped with a ballistic safety-parachute because there are no overhead rotors for the canopy to get entangled with. Its not an ideal solution, because of the potential for mechanical failure of the ejection system, and because it may require the fast reaction of the pilot to an emergency situation and is not automatic.
Previous winged vtol aircraft have relied upon two separate thrusting sources—one for lift and one for forward flight. Melkuti (U.S. Pat. No. 7,281,680B2) departed from this by using one rotary wing means for both lift and thrust, and he identified this difference as the main advantage over prior art. My Liftjet invention shares this particular feature with Melkuti, but is otherwise different from his, and all other winged vtol aircraft in every respect; To date, all winged vtols, including Melkuti, place their wing surface on the same plane as their lifting rotor, and there is a particular reason to doing so. But there are also drawbacks. As Melkuti describes . . . “Spanwise the forward lifting surface 22 terminates in wing extensions 26 and 28.” (page 3, paragraph 35). The use of such “stub wings” is common—made necessary by the structural limitations of setting the wings alongside the quite large circular hole of the rotor duct. The wings are also made small because of the design philosophy in which there are distinct and separate modes for vertical vs. horizontal flight. The argument goes, ‘because the rotor is relied upon for descent, the wings are necessary only to sustain level flight—therefor the faster, the better.’ My approach is quite different, for without the need for a hole for the rotor, my wing, or “descent vane” can be made much larger—large enough, in fact, to serve its primary duty as an emergency descent-arresting airfoil. The prevailing philosophy on winged vtol aircraft has resulted in an overall configuration of a low-wing general-aviation monoplane, with a rotor system fitted on. Again using Melkuti for example, we find a streamlined, low-slung fuselage with tandem seating, again to favour speed. My Liftjets' top speed could never compare, being that the pilot, in a standing position, produces, with or without the vertical columnar canopy, much more drag. But the Liftjet is a machine for a different purpose; By using the simple weight-shift control of a standing pilot, all other flight controls are made redundant. For other winged vtol, these controls are extremely complex, with a separate control system each for both vtol flight and for horizontal flight. Melkuti, for instance, employs 8 different push-rod runs to 8 different control surfaces, plus 9 louvers for vtol flight. Altogether these linkages contain approximately 53 pivot-pins, plus control horns, pull-rods, control sticks, etc—a system of extreme complexity, cost, weight penalty, and difficulty of operation. To date, the control system of this and all other winged vtols has been at least as complex as that of a normal light plane. But, like a hang glider, the basic Liftjet has no controls, (other than a throttle) and the advanced version has only 1 pivot hinge, which connects the descent vane to the pylon for an expanded performance envelope. So while the Liftjet may be slower, it is much easier to learn, and it has a safe way to land when the engine quits, all of which makes the Liftjet much more suited as a recreational vehicle than other winged vtols could ever be.
Safe Descent Capabilities of Existing Types of Personal Aircraft:
While no air-vehicle can be made completely safe, motorized paragliders have the highest degree of intrinsic safety, on account of its full-time, parachute-like, power-off descent capability. It is no accident that the most popular form of flying is also the safest; parasailing, where the air vehicle is towed behind a boat, for example—and has gained wide public acceptance. They are easy to operate. Control is by intuitive, body-shifting and can be learned in a matter of minutes, so the low instruction time and low insurance rates have enabled the commercialization of the parasail. The motorized paraglider field is also a burgeoning industry at this time. It shows the general trend towards low-cost minimum aircraft, with the emphasis on a low and slow, safe recreational flyer. The motorized paragliders' biggest drawback is that it is not capable of Vertical Take-offs and Landings (VToL) but only SToL (Short Take-off and Landing). The field is rife for new inventions.
Trends in the VToL/SToL Fields of Invention:
The VToL field has several identifiable areas of specific development. If one includes existing classes of powered motor gliders, parasails, and ultra light “backpack” helicopters, there is a definite focus on a basic “minimalist” aircraft—something along the lines of a tube-and fabric hang-glider, having simple weight-shift control—except they would also be capable of taking-off vertically. High power-to-weight ratios of available motors in the 15 to 60 hp range produce enough lift for lightweight personal vertical-flying vehicles. It has been deemed reasonable to define this basic ultra light category of VToL as limited to two people, and has been so treated in the Air Regulations of most western countries. Two people is approximately the upper manageable mass for a weight-shift controlled vehicle in any case.
Comparison of Small & Large Winged Ducted-Propeller VToL Vehicles:
The other end of the VToL spectrum involves high-speed, multi-passenger helicopters, gyroplanes, and winged hybrids, catering to a corporate market, and using wings with flat-bottomed or symmetrical airfoils, which obtain faster top speeds and more efficient level flight than the rotor-only variants. A variety of planforms are included in this category, including canards, annular wings, and box-wing configurations. All use ducted-fan, ducted propeller, or rotors for vertical lift and for all or part of the vehicles' forward propulsion. Some examples have used auxiliary, vertically-mounted propellers for forward propulsion. At a certain minimum forward airspeed, the wings begin to generate lift, which permits the setting of a more acute thrust-angle of the rotor, and therefore higher forward speed for the winged vehicle vs. its wingless equivalent. VToL vehicles of this type, having the lifting rotor buried within the fuselage, tend to be bulky, with a poor aspect-ratio, and the generally stubby wings appended to the fuselage produce poor gliding characteristics, with relatively high stall speeds. Stall-speeds of about 80 kph are common for this class, so—although a gliding, unpowered emergency descent is possible, it is a relatively fast glide, requiring a prepared runway of sufficient length. Non of the winged ducted-propeller VToL craft in the art exhibit slow un-powered descents that are vertical or near-vertical. By comparison, the air vehicle of this invention is equipped with descent-vanes rather than wings, and these descent-vanes are designed for the primary purpose of providing a safe, vertical or near-vertical descent in the event of an engine failure. That these descent-vanes, in some embodiments, have the shape and the appearance of wings, and may also produce additional lift during forward flight, is a secondary and incidental aspect of their design.
Comparison with Other Types of VToL Vehicles:
There is no advantage to large multi-passenger helicopters or gyroplanes being formed into, or adopting the shape of a parachute-glider, because any wing or appendage of reasonable proportion will be too small in area, in relation to the vehicle's weight, to produce a slow vertical descent. An aerodynamic descent as a secondary, emergency descent feature is appropriate for smaller craft only—one person, two person, or at a maximum, three person flying vehicles, and, with modifications to be described herein, small transport vehicles carrying several passengers or cargo.
Background of Invention—Objects and Advantages:
Accordingly, besides the objects and advantages of a ‘piggy-backed” descent-vanes to provide safe unpowered descent for a VToL air-vehicle, as described in this patent, there are several additional objects and advantages of the present invention. Any widespread recreational use of a vertically-flying personal aircraft will be predicated upon providing a vehicle that has a built-in, full-time, parachute-like descent capability. The problem to be solved is not: “High-Speed, High-Performance flight with VToL Capability”, but: “Safe, Low-Speed Flight with VToL Capability”. The art contains winged ducted-aircraft, with none claiming the benefit of a safe vertical descent from a secondary system based upon a wing or vane-like appendage that is specifically configured to provide maximum air resistance (continued . . . )
during a vertical descent. With my air-vehicle invention, while a vertical or near-vertical descent is available to the pilot at any time, in practice, a ‘gliding-descent’ is also at the option of the pilot and may be used where the size of the landing area permits.
My air-vehicle invention operates by different means of descent, in that it has a secondary, parachute/gliding descent that is controllable, and that provides the benefit of increased safety in the event of the failure of the motorized lifting-rotor. The method of operation, the structure, and the composition and materials of this air-vehicle invention's planar appendages are all dissimilar to the appendages of the prior art. My invention's appended Descent-Vanes are specifically descent-arresting surfaces, with embodiments having fabric descent panels, or semi-rigid descent-panels—materials and construction methods not possible with the conventional designs of the arts' Winged Ducted Propeller Aircraft.
Motorized Paragliders:
In the field of personal flying vehicles, trends in the art show a strong inclination towards vehicles having VToL capability. Motorized paragliders can be configured to land vertically, or almost vertically, but cannot take-off vertically. Although they are classed as SToL (Short Take-off and Landing) they have other features, such as safety, simplicity of operation and low-cost, which have made them an important segment of the art. From this perspective, the addition of a horizontal rotor for VToL flight might seem a logical next-step in the art, but the reality is that the marriage of a descent system based on a parachute-like element, together with a lifting system comprising a rotor, with the attendant problem of its downwash, is anti-intuitive. However, there is a limited set of configurations that can successfully mate the two elements.